The phase transformation and growth of mullite (3Al 2 O 3 ·2SiO 2 ) in kaolin ceramics have been investigated using X-ray diffractometer, transmission electron microscope, select area electron diffractometer, energy dispersion spectrometer and differential thermal analysis. The mullite which was transformed from kaolin appears at 1050 °C by XRD and tallies with DTA. The initial mullite crystal showed a plate-like morphology. The Al 2 O 3 content in mullite crystal increased from 49.57 to 71.37 wt.% but the lattice parameters of a , b and c axes decreased from 8.085, 8.106 and 3.215 A to 7.882, 7.974 and 2.946 A, respectively, with the grain width increasing from 20 to 70 nm when the kaolin was sintered at 1300 °C for 30 min. The nonisothermal activation energy of mullite crystallization in kaolin ceramics was 1182.3 kJ mol −1 . The growth morphology parameters n and m were both about 2.0, indicating that the bulk nucleation was dominant in mullitization and the crystal growth was controlled by diffusion. Seemingly, this study has been attempted to provide an integrative presentation of the thermal–structural characterization together with detailed kinetic and mechanistic interpretations.

Phase transformation and growth of mullite in kaolin ceramics

Microstructural evolution in a model triaxial porcelain was studied by X-ray diffractometry and electron microscopy of quenched samples after firing for 3 h at 600°–1500°C. The clay component dehydroxylated to metakaolin at ∼550°C. Metastable sanidine formed from decomposition of the feldspar at >600°C and dissolved at >900°C. Liquid formation at ∼1000°C was associated with melting of feldspar and silica discarded from metakaolin formation via the K2O–Al2O3–SiO2 eutectic. Liquid content increased at 1000°–1200°C with further feldspar melting and additionally at >1200°C because of quartz dissolution. Small (≤7 nm) mullite and γ-alumina crystals precipitated in pure clay relicts and larger (≤30 nm) mullite crystals in mixed clay-feldspar relicts at 1000°C. In the evolving microstructures, three regions were observed. These regions were derived from pure clay relicts containing primary (type-I) mullite; feldspar-penetrated clay relicts, also containing secondary (granular type-II) mullite; and the matrix of fine clay, feldspar, and quartz, containing secondary (granular type-II and elongated type-III) mullite. In addition to shape, the mullite size changed, increasing from regions containing type-I to type-III mullite, because the increasingly fluid liquid enhanced crystal growth. Below 1300°C, primary mullite was richer in Al2O3 than the secondary mullite, and the glass composition was inhomogeneous, with the K2O and Al2O3 contents varying throughout the microstructure. Above 1400°C, mullite began to dissolve.

Microstructural Evolution in Triaxial Porcelain

A common theme over the past 100 years in refractories science and technology has been to generate a protective refractories layer in a high-temperature container, often by reaction of the refractory materials with the contents (glass, slag, or atmosphere). The history of refractories during the 20th century is used in this review to illustrate how techniques-such as slag splashing, in situ spinel generation in castables, magnesia dense layer formation in magnesia-carbon brick in steelmaking, clinker adhesion in cement kilns, and viscous boundary layer generation in glass tanks-have evolved to their present status.

Evolution of in Situ Refractories in the 20th Century

Recent studies of mullite formation in clays and clay-based vitreous ceramic systems are reviewed. In such systems, mullite crystals grow in a viscous aluminosilicate liquid, as primary 2:1 mullite in pure clays and as more acicular secondary 3:2 mullite in the presence of alkali fluxes. The stoichiometry of the mullite formed is difficult to determine since the crystals are embedded in an aluminosilicate glass matrix. However, it is likely the mullites formed have a range of Al/Si ratios as the local liquid composition varies significantly due to the heterogeneity of the microstructure. More fluid (fluxed with alkali and iron oxides) aluminosilicate liquids encourage growth of mullite crystals as does firing in oxidising atmosphere due to reducing atmosphere causing formation of Fe metal so removing the fluxing iron oxides from the aluminosilicate melt.

Mullite formation in clays and clay-derived vitreous ceramics

Various forms of mullite occur in porcelains. Primary mullite forms from decomposition of pure clay. Secondary mullite forms from reaction of feldspar and clay and feldpar, clay and quartz. Which of these forms is strongly dependent on the extent of mixing of the body raw materials. Additionally, in aluminous porcelains tertiary mullite may precipitate from alumina-rich liquid obtained by dissolution of alumina filler. The size, shape and composition of the mullite crystals is controlled by the fluidity of the local liquid matrix from which they precipitate and grow, which itself is largely determined by the extent of mixing of the original powder batch.

Influence of mixing on mullite formation in porcelain

On heating, individual platelike pseudohexagonal particles of kaolinite undergo dehydroxylation forming metakaolin at about 550°C. Between 900 and 1000°C, nucleation of nanometre scale γ-Al 2 O 3 spinel crystals occurs while the original hexagonal particle morphology is retained. At higher temperatures (1200°C) cristobalite and elongated mullite crystals nucleate, and eventually at 1300°C the particles lose their face-on pseudohexagonal habit and ball up. Dense kaolinite, clay derived bodies show similar phase and morphology evolution : after 3 h at 900°C, elongated edge-on, and more equiaxed face-on, particles of metakaolin are observed. These closely packed, platelike, amorphous particles retain the individual kaolinite particle morphology. Light and dark contrast in dense bodies appears to arise from phase separation into alumina and silica rich regions. Between 900 and 1000°C nanometre crystals of γ-Al 2 O 3 spinel nucleate within these regions and, after 3 h at 1200°C, randomly orientated mullite and cristobalite crystals nucleate from the amorphous phase.

C. J. Mcconville

Papers

Microstructural evolution in fired kaolinite